The Rab Interacting Lysosomal Protein (RILP) Homology Domain Functions as a Novel Effector Domain for Small GTPase Rab36: Rab36 REGULATES RETROGRADE MELANOSOME TRANSPORT IN MELANOCYTES

Small GTPase Rab functions as a molecular switch that drives membrane trafficking through specific interaction with its effector molecule. Thus, identification of its specific effector domain is crucial to revealing the molecular mechanism that underlies Rab-mediated membrane trafficking. Because of the large numbers of Rab isoforms in higher eukaryotes, however, the effector domains of most of the vertebrate- or mammalian-specific Rabs have yet to be determined. In this study we screened for effector molecules of Rab36, a previously uncharacterized Rab isoform that is largely conserved in vertebrates, and we succeeded in identifying nine Rab36-binding proteins, including RILP (Rab interacting lysosomal protein) family members. Sequence comparison revealed that five of nine Rab36-binding proteins, i.e. RILP, RILP-L1, RILP-L2, and JIP3/4, contain a conserved coiled-coil domain. We identified the coiled-coil domain as a RILP homology domain (RHD) and characterized it as a common Rab36-binding site. Site-directed mutagenesis of the RHD of RILP revealed the different contributions by amino acids in the RHD to binding activity toward Rab7 and Rab36. Expression of RILP in melanocytes, but not expression of its Rab36 binding-deficient mutants, induced perinuclear aggregation of melanosomes, and this effect was clearly attenuated by knockdown of endogenous Rab36 protein. Moreover, knockdown of Rab36 in Rab27A-deficient melanocytes, which normally exhibit perinuclear melanosome aggregation because of increased retrograde melanosome transport activity, caused dispersion of melanosomes from the perinucleus to the cell periphery, but knockdown of Rab7 did not. Our findings indicated that Rab36 mediates retrograde melanosome transport in melanocytes through interaction with RILP.     

Spotting the right location— imaging approaches to resolve the intracellular localization of invasive pathogens

Background

A common strategy of microbial pathogens is to invade host cells during infection. The invading microbes explore different intracellular compartments to find their preferred niche.

Scope of Review

Imaging has been instrumental to unravel paradigms of pathogen entry, to identify their exact intracellular location, and to understand the underlying mechanisms for the formation of pathogen-containing niches. Here, we provide an overview of imaging techniques that have been applied to monitor the intracellular lifestyle of pathogens, focusing mainly on bacteria that either remain in vacuolar-bound compartments or rupture the endocytic vacuole to escape into the host's cellular cytoplasm.

Major Conclusions

We will depict common molecular and cellular paradigms that are preferentially exploited by pathogens. A combination of electron microscopy, fluorescence microscopy, and time-lapse microscopy has been the driving force to reveal underlying cell biological processes. Furthermore, the development of highly sensitive and specific fluorescent sensor molecules has allowed for the identification of functional aspects of niche formation by intracellular pathogens.

General Significance

Currently, we are beginning to understand the sophistication of the invasion strategies used by bacterial pathogens during the infection process- innovative imaging has been a key ingredient for this.This article is part of a Special Issue entitled Nanotechnologies - Emerging Applications in Biomedicine.     

Rab7: Role of its protein interaction cascades in endo-lysosomal traffic

Protein-protein interaction cascades are crucial for cellular signaling pathways and cell morphogenesis. Membrane traffic along the secretory and endocytic pathways is similarly governed by regulated protein-protein interactions of diverse machineries, which are inter-regulated, assembled and disassembled sequentially to drive membrane budding, vesicle transport, membrane fission and fusion. Rab7, the key regulator in endo-lysosomal trafficking investigated extensively in the past decades, is emerging to govern early-to-late endosomal maturation, microtubule minus-end as well as plus-end directed endosomal migration and positioning, and endosome-lysosome transport through different protein-protein interaction cascades. We summarize here the key protein interaction cascades of Rab7 by focusing on endo-lysosomal trafficking regulated by its interaction with HOPs, RILP, ORP1L, FYCO1 and Mon1/Sand1-CCZ1 complex.     

Structural basis for recruitment of RILP by small GTPase Rab7

Rab7 regulates vesicle traffic from early to late endosomes, and from late endosomes to lysosomes. The crystal structure of Rab7-GTP in complex with the Rab7 binding domain of RILP reveals that Rab7 interacts with RILP specifically via two distinct areas, with the first one involving the switch and interswitch regions and the second one consisting of RabSF1 and RabSF4. Disruption of these interactions by mutations abrogates late endosomal/lysosomal targeting of Rab7 and RILP. The Rab7 binding domain of RILP forms a coiled-coil homodimer with two symmetric surfaces to interact with two separate Rab7-GTP molecules, forming a dyad configuration of Rab7-RILP(2)-Rab7. Mutations that disrupt RILP dimerization also abolish its interactions with Rab7-GTP and late endosomal/lysosomal targeting, suggesting that the dimeric form of RILP is a functional unit. Structural comparison suggests that the combined use of RabSF1 and RabSF4 with the switch regions may be a general mode of action for most Rab proteins in regulating membrane trafficking.     

Rab7a modulates ER stress and ER morphology

The Endoplasmic Reticulum (ER) is a membranous organelle with diverse structural and functional domains. Peripheral ER includes interconnected tubules, and dense tubular arrays called “ER matrices” together with bona fide flat cisternae. Transitions between these states are regulated by membrane-associated proteins and cytosolic factors. Recently, the small GTPases Rab10 and Rab18 were reported to control ER shape by regulating ER dynamics and fusion. Here, we present evidence that another Rab protein, Rab7a, modulates the ER morphology by controlling the ER homeostasis and ER stress. Indeed, inhibition of Rab7a expression by siRNA or expression of the dominant negative mutant Rab7aT22?N, leads to enlargement of sheet-like ER structures and spreading towards the cell periphery. Notably, such alterations are ascribable neither to a direct modulation of the ER shaping proteins Reticulon-4b and CLIMP63, nor to interactions with Protrudin, a Rab7a-binding protein known to affect the ER organization. Conversely, depletion of Rab7a leads to basal ER stress, in turn causing ER membrane expansion. Both ER enlargement and basal ER stress are reverted in rescue experiments by Rab7a re-expression, as well as by the ER chemical chaperone tauroursodeoxycholic acid (TUDCA). Collectively, these findings reveal a new role of Rab7a in ER homeostasis, and indicate that genetic and pharmacological ER stress manipulation may restore ER morphology in Rab7a silenced cells.     

The Structure of Melanoregulin Reveals a Role for Cholesterol Recognition in the Protein's Ability to Promote Dynein Function

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Rab24 interacts with the Rab7/Rab interacting lysosomal protein complex to regulate endosomal degradation

Endocytosis is a multistep process engaged in extracellular molecules internalization. Several proteins including the Rab GTPases family coordinate the endocytic pathway. The small GTPase Rab7 is present in late endosome (LE) compartments being a marker of endosome maturation. The Rab interacting lysosomal protein (RILP) is a downstream effector of Rab7 that recruits the functional dynein/dynactin motor complex to late compartments. In the present study, we have found Rab24 as a component of the endosome‐lysosome degradative pathway. Rab24 is an atypical protein of the Rab GTPase family, which has been attributed a function in vesicle trafficking and autophagosome maturation. Using a model of transiently expressed proteins in K562 cells, we found that Rab24 co‐localizes in vesicular structures labeled with Rab7 and LAMP1. Moreover, using a dominant negative mutant of Rab24 or a siRNA‐Rab24 we showed that the distribution of Rab7 in vesicles depends on a functional Rab24 to allow DQ‐BSA protein degradation. Additionally, by immunoprecipitation and pull down assays, we have demonstrated that Rab24 interacts with Rab7 and RILP. Interestingly, overexpression of the Vps41 subunit from the homotypic fusion and protein‐sorting (HOPS) complex hampered the co‐localization of Rab24 with RILP or with the lysosomal GTPase Arl8b, suggesting that Vps41 would affect the Rab24/RILP association. In summary, our data strongly support the hypothesis that Rab24 forms a complex with Rab7 and RILP on the membranes of late compartments. Our work provides new insights into the molecular function of Rab24 in the last steps of the endosomal degradative pathway.      

Folliculin directs the formation of a Rab34–RILP complex to control the nutrient‐dependent dynamic distribution of lysosomes

The spatial distribution of lysosomes is important for their function and is, in part, controlled by cellular nutrient status. Here, we show that the lysosome associated Birt–Hoge–Dubé (BHD) syndrome renal tumour suppressor folliculin (FLCN) regulates this process. FLCN promotes the peri‐nuclear clustering of lysosomes following serum and amino acid withdrawal and is supported by the predominantly Golgi‐associated small GTPase Rab34. Rab34‐positive peri‐nuclear membranes contact lysosomes and cause a reduction in lysosome motility and knockdown of FLCN inhibits Rab34‐induced peri‐nuclear lysosome clustering. FLCN interacts directly via its C‐terminal DENN domain with the Rab34 effector RILP. Using purified recombinant proteins, we show that the FLCN‐DENN domain does not act as a GEF for Rab34, but rather, loads active Rab34 onto RILP. We propose a model whereby starvation‐induced FLCN association with lysosomes drives the formation of contact sites between lysosomes and Rab34‐positive peri‐nuclear membranes that restrict lysosome motility and thus promote their retention in this region of the cell.     

A role for Rab7 in the movement of secretory granules in cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTL) are potent killers of virally infected and tumorigenic cells. Upon recognition of target cells, CTL undergo polarized secretion of secretory lysosomes at the immunological synapse (IS) that forms between CTL and target. However, the molecular machinery involved in the polarization of secretory lysosomes is still largely uncharacterized. In this paper, we investigated the role of Rab7 in the polarization of secretory lysosomes. We show that silencing of Rab7 by RNA interference reduces the ability of CTL to kill targets. GTP-bound Rab7 and Rab interacting lysosomal protein, RILP, interact and both localize to secretory lysosomes in CTL. Over-expression of RILP recruits dynein to the membranes of secretory lysosomes and triggers their movement toward the centrosome. Together, these results suggest that Rab7 may play a role in secretory lysosome movement toward the centrosome by interacting with RILP to recruit the minus-end motor, dynein.     

The Rab-interacting lysosomal protein, a Rab7 and Rab34 effector, is capable of self-interaction

Rab-interacting lysosomal protein (RILP) has been identified as an interacting partner of the small GTPases Rab7 and Rab34. Active Rab7 recruits RILP on the late endosomal/lysosomal membrane and RILP then functions as a Rab7 effector controlling transport to degradative compartments. Indeed, RILP induces recruitment of dynein-dynactin motor complexes to Rab7-containing late endosomes and lysosomes. Recently, Rab7 and RILP have been found to be key proteins also for the biogenesis of phagolysosomes. Therefore, RILP represents probably an important factor for all endocytic routes to lysosomes. In this study, we show, using the yeast two-hybrid system, that RILP is able to interact with itself. The data obtained with the two-hybrid system were confirmed using co-immunoprecipitation in HeLa cells. The data together indicate that RILP, as already demonstrated for several other Rab effector proteins, is capable of self-association, thus probably forming a homo-dimer.